1. Field of the Invention
This invention relates to a four-wheel steering apparatus utilized as vehicle-steering means to change the angle of rear wheels as well as front wheels for changing the direction of running of the vehicle, thus permitting reduction of the turning radius of the vehicle and also ensuring the running stability or safety thereof.
2. Related Background Art
Recently, four-wheel steering apparatus for steering not only front wheels but also rear wheels in response to the operation of a steering wheel has become in use to reduce the turning radius of the vehicle so as to be able to readily change the running direction on a narrow road or ensure safety of the vehicle when changing the running direction while the vehicle runs at a high speed.
As the prior art four-wheel steering apparatus, usually there are following four types (a) to (d).
(a) A front wheel steering unit and a rear wheel steering unit are mechanically coupled to each other such that the rear wheels are steered in a fixed relation to the steering of the front wheels.
(b) A front wheel steering unit and a rear wheel steering unit are coupled to each other by hydraulic circuit to let the rear wheels be steered in a fixed relation to the steering of the front wheels.
(c) A front wheel steering unit and a rear wheel steering unit are coupled to each other by a hydraulic mechanism, but the rear wheels and front wheels can be steered independently.
(d) A front wheel steering unit and a rear wheel steering unit are coupled to each other only electrically, and the rear wheels can be steered independently of the front wheels.
Of these four different types of steering apparatus, either type (a) or (b) is adopted when it is necessary to provide a large steering angle to the rear wheels for reducing the turning radius of the vehicle, and either type (c) or (d) is usually adopted when it is not necessary to provide a large steering angle to the rear wheels.
In the four-wheel steering apparatus of either type (c) and (d), in which the rear wheels can be controlled independently of the front wheels, there is hazardousness when the rear wheels are steered greatly while the vehicle is running at a high speed. The four-wheel steering apparatus of either type (a) or (b) is free from such hazardousness because the front and rear wheels are steered in an interlocked relation to one another in response to the driver's action.
However, the prior art four-wheel steering apparatus of either type (a) and (b), can not sufficiently improve the safety during high speed running because the front and rear wheels are steered only in an interlocked relation to one another in response to the driver's action.
More specifically, in the steering system in which the front and rear wheels are steered in an interlocked relation to one another, when giving a small steering angle to the front wheels the same phase is provided to the front and rear wheels (i.e., the front and rear wheels are turned in the same direction) for a stable lane change or the like during high speed driving, while when giving a large steering angle to the front wheels opposite phases are provided to the front and rear wheels (i.e., the front and rear wheels are turned in opposite directions) for reducing the turning radius of the vehicle and thus facilitating the garaging of the vehicle or the like. However, the apparatus may fail to ensure sufficient stability of the vehicle (although it can ensure far superior safety as compared with the case of the prior art two-wheel steering apparatus) in case when there occurs an unexpected situation during high speed driving.
For example, during high speed driving the vehicle may experience sidewise winds, the rear wheels may slip in a transverse direction at the time of high speed cornering, or sudden steering (i.e., quick handling) may be required by an unexpected occurrence such as someone or something coming into the vehicle's path. In such cases, the stability of the vehicle can be ensured if the rear wheels can be steered at a small amount. However, in this case, the rear wheels must be steered independently of the front wheels. With the apparatus of either type (a) or (b), such measure can not be effected because the rear wheels can not be steered independently of the front wheels.
While the four-wheel steering apparatus of either type (c) or (d) can provide such measure, it can not provide a considerably large steering angle to the rear wheels in view of the safety and hence can not permit considerable reduction of the turning radius of the vehicle, as noted before.